The invention relates generally to electro-optical devices, and, more particularly, to a polarity-independent optical receiver and a method for fabricating same.
Optical receivers are useful for detecting light and converting the detected light to a corresponding electrical signal. The electrical signal may be measured, and thus, the intensity of the light can be determined. When detecting and measuring coherent light using interferometric or heterodyne techniques, both the interferometric or heterodyne mixing terms as well as the direct intensity of the light are detected. A portion of the direct intensity of the light includes noise. The direct intensity detection can interfere with the desired heterodyne mixed signal, especially when the intermediate frequency (IF) falls within the direct-detection bandwidth.
One manner in which to reduce the amount of noise is to use an optical receiver having two photodetectors (e.g., photodiodes) arranged in a head to toe configuration as shown in FIG. 1. The photodiode configuration of FIG. 1 has been used with PIN-type photodiodes to suppress the common-mode intensity noise when the photodiodes are illuminated by light having a differential signal, wherein a common-mode intensity noise appears at their respective light terminals.
PIN-type photodiodes are ones in which an intrinsic region separates a p-type material from an n-type material. The operation of PIN-type photodiodes is known to those having ordinary skill in the art. In FIG. 1, optical receiver 1 includes photodiode 6 and photodiode 7 configured in a balanced detector configuration. When a negative voltage is applied to terminal 2 and a positive voltage is applied to terminal 4, both photodiodes 6 and 7 become reverse biased. Upon the application of light, denoted by hxcexd, to the absorption regions associated with photodiodes 6 and 7, the photodiodes will begin to generate an electrical current proportional to the intensity of light impinging on the photodiodes. Photodiodes 6 and 7 are said to be configured in a balanced detector arrangement because when light is applied to photodiode 6, an electrical current will be generated between terminal 2 and terminal 3, and when light is applied to photodiode 7, an electrical current will be generated between terminal 4 and terminal 3. Terminal 3 is the virtual common-mode ground and is the output terminal of the optical receiver 1. If an equal intensity of light is received by photodiodes 6 and 7, then current will flow between terminals 2 and 4, however, no current will appear at terminal 3, the virtual common-mode ground. This condition is preferable in situations in which it is desirable to suppress common-mode intensity noise corresponding to the light applied to photodiodes 6 and 7.
This balanced detector arrangement is useful in optical interferometric systems in which the detected optical signals are differential. Differential optical signals are those that are out of phase with each other, but the noise associated with them has an in-phase component. In the balanced detector arrangement, the common-mode intensity noise is canceled at the virtual common-mode ground terminal 3, thus effectively suppressing the common-mode intensity noise.
A drawback with this configuration is that the photodiodes must be biased by the application of a voltage at a certain polarity as indicated in FIG. 1. This prevents the application of bias chopping or gating where polarities at terminals 2 and 4 are periodically reversed. Bias chopping is important to translate low-frequency detected signals to higher frequencies to reduce the effect of low-frequency electronic noise. Forward biasing the conventional arrangement shown in the prior art of FIG. 1 destroys the photodiodes. Therefore, with this type of photodetector design, careful bias structures must be implemented to avoid damage to the photodiodes.
Furthermore, the voltage polarity will differ from one photodiode to the next depending on whether the photodiode is of a p-type material side up configuration or a p-type material side down configuration. This polarity confusion may lead to rework of the optical receiver if it is incorrectly biased, and possibly destruction of the device due to the application of incorrect bias voltage polarity.
Therefore, there is a need in the industry for a polarity-independent optical receiver having a balanced detector arrangement.
The invention is a polarity-independent optical receiver having a balanced detector arrangement and a method for fabricating same.
In architecture, the invention can be conceptualized a polarity-independent optical receiver, comprising a first pair of photodiodes serially connected between a first terminal and a common terminal, a second pair of photodiodes serially connected between a second terminal and the common terminal, wherein each of the photodiodes in the first pair and the second pair is oppositely oriented such that a polarity-independent bias voltage may be applied to the first terminal and the second terminal.
The invention can also be conceptualized as a method for making a polarity-independent optical receiver, the method comprising the steps of forming a first pair of photodiodes serially connected between a first terminal and a common terminal, forming a second pair of photodiodes serially connected between a second terminal and the common terminal, and oppositely orienting each of the photodiodes in the first pair and the second pair such that a polarity-independent bias voltage may be applied to the first terminal and the second terminal.
An advantage of the invention is that it provides a polarity-independent optical receiver having a balanced detector arrangement.
Another advantage of the invention is that it removes uncertainty when biasing an optical receiver.
Another advantage of the invention is that allows polarity-independent biasing of an optical receiver.
Another advantage of the invention is that it is simple in design and easily implemented on a mass scale for commercial production.
Other features and advantages of the invention become apparent to one with skill in the art upon examination of the following drawings and detailed description. These additional features and advantages are intended to be included herein within the scope of the invention.